1. Field of the Invention
The present invention relates to a photovoltaic cell for converting solarlight energy into electrical energy and, more particularly, a manufacturing method to enhance conversion efficiency of a photoelectric conversion device such as a thin film photovoltaic cell, which employs a CdTe thin film formed on a transparent substrate such as a glass substrate as a light absorber layer.
2. Description of the Prior Art
It has been well known that CdTe has a high absorption coefficient of more than 10.sup.4 cm.sup.-1 and its thin film of about 5 .mu.m thickness is capable of sufficiently absorbing the solarlight. The CdTe thin film is promising as material for the thin film photovoltaic cell since it is easy to form a polycrystal film of high quality by virtue of various thin film forming methods such as a printing method, a plating method, an evaporation method, etc. In addition, a band gap of CdTe (.about.1.47 eV) is most suitable for solarlight spectrum amongst various materials for the photovoltaic cell, and thus the highest conversion efficiency can be expected. It has also been calculated that theoretical efficiency of a CdTe thin film photovoltaic cell is in excess of 20%. However, the highest value of the conversion efficiency of the CdTe thin film photovoltaic cell which has been reported up to now is about 15%, which is largely different from a theoretical value. Like the above, the photovoltaic cell employing the CdTe thin film as the light absorber layer has been expected as the low cost and high efficiency photovoltaic cell, but it is difficult under the existing circumstances to manufacture the photovoltaic cell which employs the CdTe thin film having sufficiently high conversion efficiency as the light absorber layer with good reproducibility.
As the photovoltaic cell employing the CdTe thin film as the light absorber layer in the prior art, a pn junction photovoltaic cell is common which is formed by depositing a p-type CdTe layer 4 on an n-type CdS layer 3, as shown in FIG. 1. Though illustrated upside down in FIG. 1, a transparent conductive film 2 such as an indium-tin oxide film (ITO film) is formed on a glass substrate 1, then the n-type CdS layer 3 is formed 0.1 to 10 .mu.m thick thereon, then the p-type CdTe layer 4 is formed 1 to 10 .mu.m thick thereon, and then an ohmic electrode 5 made of Cu/Au, etc. is formed thereon. If a sheet resistance of the CdS layer 3 is sufficiently small, the transparent conductive film 2 may be omitted.
In the prior art, the CdTe/CdS photovoltaic cell shown in FIG. 1 has been manufactured according to manufacturing steps described in the following. That is, the cell has been manufactured according to following steps (1) to (5) (this is called "a first manufacturing method in the prior art" hereinafter):
(1) The transparent conductive film 2 such as the indium-tin oxide (ITO) is deposited by the sputtering method, etc. on the glass substrate 1 such as a Corning 7059 substrate to have a thickness of about 150 nm to 1 .mu.m such that a sheet resistance of less than 10 .OMEGA./.quadrature. can be given.
(2) The n-type CdS layer 3 is deposited by the vacuum evaporation method, etc. at a substrate temperature 350.degree. C. to have a thickness of 0.1 to 10 .mu.m. If a sheet resistance of the CdS layer 3 is enoughly small, there is no necessity of the above transparent conductive film 2.
(3) Next, either CdTe molecules or Cd and Te with corresponding mole fraction are deposited 1 to 10 .mu.m thick by means of a screen printing method, an electrolytic plating method, a spray method, or the like.
(4) Cadmium chloride (CdCl.sub.2) or chlorine (Cl.sub.2) is then mixed with or added to such CdTe molecules or Cd and Te with corresponding mole fraction in proper quantity. In turn, a resultant structure is annealed in an air or an inert gas at a temperature of 350 to 700.degree. C. for about 0.1 to 2 hours to thus obtain the p-type CdTe layer 4 which has a substantially equal stoichiometry.
(5) Finally, the ohmic electrode 5 is formed on the p-type CdTe layer 4. For instances a composite film made up of a 10 nm thick Cu layer and a 100 nm thick Au layer may be used as the ohmic electrode 5.
The CdTe/CdS photovoltaic cell shown in FIG. 1 can also be manufactured according to following manufacturing steps (this is called "a second manufacturing method in the prior art" hereinafter). That is, according to the second manufacturing method in the prior art, the CdTe/CdS photovoltaic cell has been manufactured based on following steps (1) to (5):
(1) The transparent conductive film 2 such as the ITO film is deposited about 200 nm thick by the sputtering method, etc. on the glass substrate 1 such as the Corning 7059 substrate.
(2) The n-type CdS film layer is deposited by the vacuum evaporation methods etc. at a substrate temperature 350.degree. C. to have a thickness of 0.1 to 10 .mu.m, e.g., a thickness of 0.15 .mu.M.
(3) A p-type CdTe layer is formed by the vacuum evaporation method at a substrate temperature 350.degree. C. to have a thickness of about 4 .mu.m.
(4) The p-type CdTe layer is dipped in a methanol (CH.sub.3 OH) solution containing copper chloride (CuCl.sub.2) or a CH.sub.3 OH solution containing CuCl.sub.2 and CdCl.sub.2, then is dried by natural drying, and then is annealed at 400.degree. C. for 15 minutes in an N.sub.2 +O.sub.2 (4:1) atmosphere.
(5) A surface of the CdTe layer is etched by using a K.sub.2 Cr.sub.2 O.sub.7 +H.sub.2 SO.sub.4 +H.sub.2 O solution, etc., then Cu (10 nm)/Au (100 nm) are deposited by the vacuum evaporation, and then annealed at 150 .degree. C. for three hours in the air, whereby resulting in the ohmic electrode 5.
In the prior art, it has been appreciated that the cadmium chloride (CdCl.sub.2), the copper chloride (CuCl.sub.2), or the chlorine (Cl.sub.2) which is used in the step (4) in the above first and second manufacturing methods can provide such advantages that the CdS layer 3 and the CdTe layer 4 serving as polycrystal films are grown in grain size several times to several tens times larger than before, conductivity of the CdTe layer 4 is changed into p-type conductivity so as to form a pn junction between the n-type CdS and the p-type CdTe, and interfacial diffusion through the CdS/CdTe junction is facilitated to form a gradient composition layer so that generation of the defects due to the lattice mismatching can be prevented. In this manner, the case is popular in the methods in the prior art where either chloride such as cadmium chloride or chlorine is introduced during deposition of either the CdTe molecules or Cd and Te with corresponding mole fraction. In this case, in order to enhance photoelectric conversion efficiency of the CdTe photovoltaic cell, the chloride or chlorine has to be controlled to its optimum amount according to respective different deposition methods. The optimum amount must not be too much reduced nor increased, and it must be controlled to proper quantity.
In the above method of manufacturing the CdTe/CdS photovoltaic cell in the prior art, there has been such a problem that, when the CdS layer 3 and the CdTe layer 4 are grown in grain size approximately ten times in the above step (4), strain is caused at an interface between the transparent conductive film 2 and the CdS layer 3 and subsequently this strain causes another strain at the CdS/CdTe junction interface, thereby decreasing an open-circuit voltage Voc and a fill factor FF of the CdTe photovoltaic cell. Hence it is difficult to manufacture the CdTe/CdS photovoltaic cell having sufficiently high conversion efficiency with good reproducibility.
Although, as will be described in detail later, the present invention includes formation of a thin film such as an indium oxide film (In.sub.2 O.sub.3 film), a tin oxide film (SnO.sub.2 film), or the like between a conductive substrate and an n-type semiconductor layer as one of features, techniques have been set forth in Patent Application Publications (KOHYOs) 8-500209 and 8-500210 wherein a high conductivity conductive layer such as tin oxide and a low conductivity conductive layer are formed on the glass substrate and then an n-type semiconductor layer (n-type CdS layer) is formed thereon. However, in order to improve quantum efficiency with respect to short wavelength light, these techniques have been employed to make the CdS layer extremely thin and therefore such techniques substantially differ from the present invention. In other words, the low conductivity conductive layers which have been set forth in Patent Application Publications (KOHYOs) 8-500209 and 8-500210 are associated with a technique to avoid a disadvantage caused by pinholes, etc. generated in the CdS layer when the CdS layer is made extremely thin less than 50 nm. More particularly, the low conductivity conductive layers are needed as an n-type backup heterojunction material to avoid short circuit of the photovoltaic cell due to the pinholes, etc. in the CdS layer whereas the high conductivity conductive layers are respectively such layers that are necessary to make ohmic contact between the low conductivity conductive layer and the electrode layer of the photovoltaic cell. In these documents, an electric power is in fact generated between the low conductivity conductive layer and the p-type CdTe layer and thus there are descriptions to the effect that the CdS layer may be removed completely. Accordingly, it can be understood from these descriptions that the techniques set forth in the above Patent Application Publication (KOHYO) 8-500209, etc. are different techniques from the present invention, which have objects, functions, and effectivenesses being different from those of the present invention.